def avg(a):
    b={}
    for key in s:
        sum1=0
        for i in range(0,3):
           sum1+=a[key][i]
        avg="{:.2f}".format(sum1 / 3)
        b[key]=avg
    return b    
s={'小李':[77,54,57],'小张':[89,66,78],'小陈':[90,93,80],'小杨':[69,58,93]}
print('这位学生3科的平均成绩为',avg(s))
